Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber. Some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the cable transitions cause impedance mismatches and non-linear changes in the cable characteristics. These abrupt changes in characteristics cause some of the transmitted signal to be reflected. The ratio of energy bounced back depends on the impedance mismatch. Impedance discontinuities cause attenuation, attenuation distortion, standing waves, ringing and other effects because a portion of a transmitted signal will be reflected back to the transmitting device rather than continuing to the receiver, much like an echo. This effect is compounded if multiple discontinuities cause additional portions of the remaining signal to be reflected back to the transmitter. This is a fundamental problem with the daisy chain method of connecting electronic components.
Superconducting microwave circuits have similar problems caused by circuit discontinuities and in particular when propagating signals between conventional circuits residing in room temperatures and superconducting circuits residing in superconducting cooled cryogenic temperatures (e.g., 4° K), referred to as ‘cold space’. That is couplings of the circuits and splitting and combining of microwave signals result in impedance mismatches due to the circuit transitions, and as a result cause signal reflections of AC propagated signals that distort the original AC signal (e.g., clock signal) from propagating through the microwave circuit. Some attempts have been made to provide for impedance matching of microwave signal paths that have their disadvantages.
For example, FIG. 1 illustrates one conventional microwave circuit 10 having a superconducting circuit 12 that employs an AC source (VAC) to provide an AC input signal to the superconducting circuit 12. The AC source VAC is source terminated by an input termination resistor RTIN. Both the input termination resistor RTIN and the AC source VAC reside in a room temperature environment coupled to the superconducting circuit 12 via, for example, a coaxial cable. The superconducting circuit 12 resides in a cold space 16, such as a supercooled cryogenic refrigerator. The superconducting circuit 12 includes a power splitter 14 that splits the AC input signal into eight AC intermediate signals that propagate through the superconducting circuit via superconducting transmission lines 18. Each of the eight superconducting transmission lines are terminated by a respective termination resistor (RTOUT1 through RTOUT8) that also resides in the cold space 16. The problem with this configuration is that the termination resistors RTOUT1 through RTOUT8 dissipate power into the cold space, and it takes a great deal more power than the dissipated power to maintain the supercooled cryogenic temperatures in the cold space as a result of the power dissipated by the termination resistors RTOUT1 through RTOUT8. For example, it may take up to 1000 watts of power to keep the cold space at the selected cryogenic temperature for a power dissipation of a single watt in the cold space.
FIG. 2 illustrates another microwave circuit 30 having a superconducting circuit package 32 with a single output termination resistor RTOUT at room temperature. The superconducting circuit package 32 includes a splitter 34, a Reciprocal Quantum Logic (RQL) circuit 36 having a plurality of superconducting transmission lines 40 and other superconducting circuitry (e.g., bias inductors), and a power combiner 38 residing on superconducting circuit package 32. The superconducting circuit package 32 can be a printed circuit board that resides in a cold space. An AC source (VAC) is terminated by an input resistor (RTIN), which both reside at room temperature. The AC source VAC provides an AC input signal to the power splitter 34 that splits the AC input signal into a plurality of AC intermediate signals that are applied to inputs of corresponding superconducting transmission lines 40 of the RQL circuit 36. The plurality of AC intermediate signals propagate through the plurality of superconducting transmission lines 40 to respective outputs coupled to inputs of the power combiner 38. The power combiner 38 combines the plurality of AC intermediate signals into a single combined AC output signal to be terminated by an output termination resistor (RTOUT) that resides outside the cold space in room temperature.
However, the transitions from the AC input source VAC to the power splitter 34, transitions from the superconducting transmission lines to the power combiner 38, and transitions from the power combiner 38 to the output termination resistor RTOUT may cause reflections in the superconducting circuit. In particular, the reflections from the transitions to and from the power combiner 38 as shown by the dashed lines cause standing waves in the RQL circuit 36. The standing waves consist of the desired forward traveling wave of amplitude A, and the undesired backward traveling wave of amplitude B, as shown in the equation of the AC input signal, Aeiωt+ Be−iωt. The standing waves directly decrease operating margins in the superconducting circuit package 32.